This invention generally relates to decorative laminates and methods of producing the same. More particularly, this invention relates to high-wear, no-overlay decorative laminates employing a fully hydrolyzed polyvinyl alcohol modified melamine-formaldehyde resin coating in place of an overlay sheet.
Conventionally, decorative laminates are made of three essential layers: a core layer, a print layer, and a surface layer. The core layer constitutes a bottom or supporting layer onto which the other layers are bonded. In normal high-pressure laminate manufacture the core layer consists of a plurality of cellulosic sheets. The core sheets are generally made from a kraft paper impregnated with a laminating resin. Laminating resins commonly used for the core layer include phenolic, amino, epoxy, polyester, silicone, and diallyl phthalate resins to name but a few. The industrially preferred laminating resin for decorative laminates appears to be a phenolic resin made from the reaction of phenols with formaldehyde. In low-pressure laminate manufacture the core layer is generally comprised of a sheet of particleboard ranging from 3/8" to 1" in thickness.
Placed above the core layer is the print layer which is generally an alpha cellulose pigmented paper containing a print, pattern, or design that has been impregnated with a melamine-formaldehyde resin. Typically, the printing is performed prior to impregnation by a high-speed rotogravure.
The cured melamine-formaldehyde resins are colorless and resistant to light; they are resistant to a variety of solvents and stains; and their heat resistance makes them immune to burning cigarettes, boiling water and heated containers up to about 325.degree. F. Without these melamine-formaldehyde resins the decorative laminate industry would not exist as it is known today. However, because these resins are extremely brittle, they sometimes require reinforcement.
The surface layer, or overlay as it is commonly referred to, is a high-quality alpha cellulose paper impregnated with a melamine-formaldehyde resin. This layer protects the print sheet from external abuse such as abrasive wear and tear, harsh chemicals, burns, spills and the like. It is primarily the melamine-formaldehyde resin which accounts for these protective properties. The alpha-cellulose paper acts as a translucent carrier for water-thin resin, imparts strength to the rather brittle melamine-formaldehyde resin, maintains a uniform resin thickness in the overlay by acting as a shim, and controls resin flow.
The core layer, print layer and surface layer are stacked in a superimposed relationship, between polished steel plates and subjected to a pressure and temperature for a time sufficiently long enough to cure the laminating resins impregnating the respective layers. The elevated temperatures and pressure actually cause the impregnated resins within the sheets to flow which consolidates the whole into an integral mass, known as the laminate. These laminates find use as counter tops, table tops, furniture, store fixtures and the like.
Employment of the overlay sheet has, however, generated a host of unwanted problems. The overlay contributes substantial material cost to the manufactured laminate. Not only are there raw material costs involved, such as expensive high-quality alpha cellulose paper and melamine-formaldehyde resins, but ther exists substantial processing costs, such as collating sheets, scrap losses generated by the brittle and difficult-to-handle impregnated sheets, as well as the cost of impregnation itself. In addition to these unwanted expenditures and processing steps, the translucent character of the overlay sheet becomes visually disturbing in decorative laminates when, through the use of more advanced printing techniques, a sharper, more intricate design is employed.
Thus, there exists the need for substitution of the overlay sheet by a tough and transparent thermoset layer that will eliminate haze or blur, incur fewer processing steps and lower costs, and yet maintain sufficient resistance to external abuse to be commercially acceptable. The provision for such a layer would fulfill a long-felt need and constitute a significant advance in the art.